The invention relates to molded polyolefin parts of improved thermal stability at elevated temperatures and improved stiffness, made by blow molding or injection molding from mixtures of unmodified propylene polymers and modified propylene polymers, which are suitable for use in the packaging industry, in the domestic appliances industry, to meet the requirements of laboratories and hospitals, for gardening and agricultural implements, for transporting containers and for components in the automobile industry and components of machinery and electric and electronic equipment. Furthermore, the invention relates to a method for producing these molded polyolefin parts.
Molded polyolefin parts, produced by blow molding or injection molding, are known.
The production of molded polyolefin parts, produced by blow molding from polyethylene, has the advantage over the production of parts, produced by blow molding from polypropylene, in that parts having a large volume can also be produced from polyethylene. The low values for the dimensional stability at elevated temperatures, the stiffness, the stress crack resistance, the transparency and the surface hardness are disadvantages of blow-molded polyethylene parts.
Blow-molded polypropylene parts in the form of bottles, coverings, connecting pieces, boxes and small-capacity containers are known ("Blasformen von Polypropylen" (Blow-Molding Propylene) VDI-Verlag Dusseldorf 1980; Modem Plastics Intern. (1996) 5, 55-57; Nei.beta.l, W. Kunststoffe 82 (1992) 140-142).
Polypropylene blends with talcum or glass fibers to further increase the stiffness and dimensional stability at elevated temperatures of the blow-molded parts and polypropylene blends with elastomers to increase the cold impact strength of the blow molded parts are also known (Lee, N., "Plastic Blow Molding Handbook", Van Norstrand Reinhold Publishers, New York 1990, pages 391-395).
The more nonuniform distribution of wall thicknesses as the volume of the blow molded parts increases is a disadvantage of blow molded polypropylene parts. Large volume parts can be produced from polyethylene, but not from polypropylene.
Known methods for producing blow-molded polyolefin parts are extrusion blow molding, extrusion stretch blow molding, injection blow molding and injection stretch blow molding (Lee, N., "Plastic Blow Molding Handbook", Van Norstrand Reinhold Publishers, New York 1990; Rosato, D., "Blow Molding Handbook", Carl-Hanser-Verlag Munich 1989).
The extrusion blow molding represents a two-step process, for which a tube is produced as parison in the first step and is molded in the second step in a synchronously operating blowing station in a divided mold and molded by being blown up into the hollow object.
Extrusion stretch blow molding is a variation of the extrusion blow molding, in which the preformed hollow object is subjected in a further step of the method in a blowing station to longitudinal stretching by a stretching stamp and radial stretching by an air blast.
In injection blow molding, the hollow object parison is produced by injection molding. The parisons are supplied either discontinuously to separate blow-molding equipment and molded or, in the multi-station injection blow molding equipment, by a turning table to the blowing mold and, after molding, to the ejector. In the case of injection blow molding as a variation of injection blow molding, an additional longitudinal stretching by a stretching stamp takes place in the blowing station during the molding process.
Known methods of increasing the stiffness of molded polyolefin parts, which are produced by injection molding and based on polypropylene, are the use of polypropylene blends with fillers, such as talcum (Grolik, W., Kunststoffe 80 (1990) 3, 342-347) or wollastonite (Paakonen, E., Kunststoffe 77 (1987) 6, 602-606), or the use of polypropylene blends with glass fibers (Zettler, M. Kunstoffe 79 (1989) 9, 797-803).
Furthermore, it is known that the stiffness of injection molded polypropylene parts can be increased by using nucleating agents, such as sodium benzoate or sorbitol derivatives (Pukansky, B., ANTEC '96, 2317-2321).
The simultaneous decrease in impact strength of the injection molded parts when fillers, glass fibers or nucleating agents are used to increase the stiffness of the injection molded polyolefin parts is a disadvantage.
It is also known that the addition of ethylene-propylene elastomers improves the toughness of talcum-filled polypropylene (Pukanszky, B., Polymer Engineering and Science 35 (1995) 24 1962 1971) and the addition of acrylic acid-modified ethylene-propylene elastomers improves the toughness of glass fiber-reinforced polypropylene (Keiner, I., Angew. Makromoleculare Chem. 189 (1991), 207-218).
The addition of elastomers is, however, associated with a loss in stiffness and of dimensional stability at elevated temperatures, so that a balanced profile of the properties of stiffness and dimensional stability at elevated temperatures of injected molded parts subjected to high stresses cannot be achieved by these measures.
Finally, stiff and tough injection-molded parts, based on mixtures of propylene-ethylene block copolymers and ethylene-propylene elastomers, are known (JP 9703295, JP 9703298). For these reactor blends, however, the stiffness level is unsatisfactory.